In the fabrication of multilayer ceramic substrates for packaging semiconductor devices, conductive metal patterns are screened on individual ceramic green sheets by printing, such as, extrusion printing, using mask, such as, a metal mask, placed in contact with the green sheet. The screen printing method involves squeeging paste onto the green sheet surface using a mesh mask or an emulsion mask. After screening, the green sheets are assembled and aligned, and laminated followed by a sintering operation to form a multilayer ceramic substrate. Fabrication techniques for such substrates, including design, screening equipment, and paste screening process are well known in the art.
However, advance ground rule electronic packaging requires printing closely spaced conductive metal pattern on a substrate, and using a screening mask that has highly dense fine dimension etched features. It has been observed that such screening masks have the problem of paste residue entrapment in the mask features in addition to a surface residue when the paste is screened to deposit conductive pattern onto the green sheets. This requires that the metal mask be cleaned after one or more screening passes to eliminate/minimize the possibility of defects in subsequently screened pattern. Any defects in the screened paste pattern replicates into the final product causing yield losses. On the other hand, mesh masks can be used for multiple screening passes, typically 100-200 green sheets may be screened using the same mesh mask before it requires cleaning and therefore, the cleaning frequency for a mesh mark is relatively low in comparison to a metal mask.
Conductive pastes used in screening processes for the delineation of wiring, vias, I/O (input-output) pads, and other pattern features comprise metal particles with or without added inorganic particulate fillers. These constituents are mixed with an organic binder and solvent vehicle along with wetting agents, dispersants/surfactants, plasticizers, and other additives such as thickening agents, antioxidants, and coloring agents which are well known in the fabrication of electronic components.
Most commonly used conductive pastes in multilayer ceramic fabrication are based on molybdenum or tungsten metal powders dispersed in an organic polymer binder, such as, for example, ethyl cellulose, polymethylmethacrylate, and the like, or polyhydrocarbon based thermoplastic resins in a high boiling solvent vehicle.
Other conductive pastes that can be used for screening processes in multilayer ceramics could comprise copper, gold, nickel, tin, to name a few. Because of the variety and complex chemical make-up of polymer-metal pastes, it is required that the mask cleaning medium and process selected be such that it provides complete and efficient cleaning of all types of pastes from screening masks and associated equipment.
In automated processes for high volume production of multilayer ceramic substrates, it is also necessary that the speed of mask cleaning be compatible with the cycle time pre-set by the thruput requirement and other dependent operations.
Recently, several water-based cleaners have become commercially available and many aqueous detergent compositions have been described for cleaning/degreasing purposes, for example, printed circuit board assemblies and other parts in device fabrication to remove soldering flux, oil/grease, and other organic residues invariably formed during bonding and assembly processes in microelectronics. These are generally based on a combination of surfactants in water and/or alkaline detergent compositions comprising alkali metal salts, such as, sodium metasilicate, sodium carbonate, tribasic sodium phosphate, sodium tripolyphosphate, and combinations thereof, as well as highly alkaline solutions based on alkali metal salts, alkali metal hydroxides, and mixtures thereof with alkanolamines. Aqueous cleaner formulations comprising this category of alkaline detergents are described in U.S. Pat. No. 5,234,506 (Winston) and U.S. Pat. No. 5,264,047 (Winston) for removing soldering flux, wax, and grease from printed circuit boards.
One of the major considerations with aqueous cleaning is wastewater management for compliance with the Clean Water Act regulations, which requires that all possible sources of aquatic impact and other environmentally undesirable constituents be removed from the effluent before discharge to surface waters. This requires special and costly wastewater treatments, such as, precipitation with complexing agents to form sludge that carries active ingredients of cleaning compositions, metals, and other dissolved constituents, and which results in solids loading to the waste. To minimize waste and reduce cost with conservation of water in aqueous cleaning, there is currently significant industry direction toward environmentally suitable zero waste processes by devising a methodology for reclamation, recycling, reuse, etc.
U.S. patent application Ser. No. 09/096,840, filed on Jun. 12, 1998, entitled "REMOVAL OF SCREENING PASTE RESIDUE WITH QUATERNARY AMMONIUM HYDROXIDE-BASED AQUEOUS CLEANING COMPOSITIONS", assigned to the assignee of the instant patent application, and the disclosure of which is incorporated herein by reference, discloses the use of quaternary ammonium hydroxide-based aqueous cleaning compositions as a more environmentally friendly medium for removing screening paste residue.